Conventionally, butadiene rubber has been widely used for forming various rubber parts. Butadiene, the raw material of butadiene rubber, is produced as a by-product in producing ethylene by means of naphtha cracking. In recent years, as methods for producing ethylene, those in which natural gas such as ethane is used as the raw material are spreading, a reduction in the production of butadiene is predicted. Thus, use of synthetic rubbers not including butadiene as the raw material as an alternative material for butadiene rubber has been investigated in various ways.
One example of synthetic rubbers that have been investigated as alternative materials for butadiene rubber is a monocyclic olefin ring-opening polymer (e.g., a cyclopentene ring-opening polymer or cyclooctadiene ring-opening polymer), which can be obtained by ring-opening polymerizing a monocyclic olefin. For example, Patent Document 1 discloses a rubber composition for tires that contains a cyclopentene ring-opening polymer, a solution-polymerized styrene-butadiene rubber, and silica. In accordance with the technique of this Patent Document 1, the composition is suitable for a tire application in which low heat generation properties and cold resistance are required, but is not necessarily suitable for applications in which lower temperature use is required.